SYDNEY, Australia — President Trump has said he plans to nominate Adm. Harry B. Harris Jr., the commander of the United States Pacific Command and a vocal critic of China’s military expansion in the region, as ambassador to Australia.

Admiral Harris, 61, who has spent 39 years in the Navy, is expected to easily win confirmation in the Senate.

As China flexes its might and North Korea speeds its nuclear program, a strong alliance between the United States and Australia is seen by many as crucial to maintaining the balance of power in the Asia-Pacific region.

Here’s a look at the man who will most likely become central to that relationship.

Who is Admiral Harris?

Born in Japan to a Japanese mother and an American father, he was the first Asian-American appointed to lead a combat command.

As head of the Pacific Command, he oversees about 375,000 military and civilian personnel, conducting operations in a region that spans more than 100 million square miles, or more than half of the earth’s surface.

Where does he stand on regional issues?

China

Appointed to lead the Pacific Command President Obama, Admiral Harris has taken a hard line against Chinese military action, calling Beijing’s policy to build bases in the South China Sea “provocative and expansionist.”

He has described China’s efforts to build artificial islands in contested waters “a Great Wall of sand.” Since his appointment, he has advocated actively patrolling the South China Sea in so-called freedom of navigation operations.

North Korea

As much as Admiral Harris has warned of Chinese aggression, he has made clear that he believes the biggest threat to the region is North Korea.

“Our most volatile and dangerous threat is North Korea, with its quest for nuclear weapons and the means to deliver them intercontinentally,” hesaid during a 2014 Senate confirmation hearing.

“Our collective efforts amid the challenges,” he said, “underscore the importance of America remaining a strong and engaged in the region.” He continued, “American leadership does matter.”

Despite those warnings, Admiral Harris has made clear that the solution to the North Korean threat is not necessarily though the use of American force.

In a statement last year, he said it was crucial that the United States “bring Kim Jong-un to his senses, not his knees.”

What would his appointment mean for Australia?

It has been more than a year since the United States had an ambassador in Australia, and Admiral Harris’s nomination comes amid fears that President Trump was abandoning the countries’ traditional alliance.

Admiral Harris’ nomination “is a demonstration that that’s not actually happening,” said Peter Jennings, executive director of the Australian Strategic Policy Institute.

Mr. Turnbull, the prime minister, welcomed news of the nomination on Twitter, saying on Twitter, “Look forward to seeing you in Canberra, Harry.”

Admiral Harris has frequently visited Australia and has said he believes the alliance between the two countries “anchors peace and stability in the region.”

“Australia,” he added, “plays a leading role in regional security, capacity-building efforts and addressing disaster response.”

But how Admiral Harris handles Australia’s relationship with China is an open question. Australia increasingly finds itself economically dependent on China, its largest trade partner, even as some politicians have warned that the rising Asian power is trying to influence Australian politics.

“What he needs to be aware of is the sensitivity around looking like Australia is doing America’s bidding,” said Euan Graham, director of the international security program at the Lowy Institute.